In computer networking, a Media Access Control (MAC) address is a unique identifier assigned to network adapters or network interface cards (NICs) usually by the manufacturer for identification. A MAC address may also be known as an Ethernet Hardware Address (EHA), hardware address, adapter address, or physical address. MAC addresses are used in the Media Access Control protocol sub-layer of the OSI reference model.
MAC address learning is a service that characterizes a learning bridge, in which the source MAC address of each received packet is stored so that future packets destined for that address can be forwarded only to the bridge interface on which that address is located. The bridge domain learns unicast MAC addresses to avoid flooding the packets to all the ports in the bridge domain.
The implementation of MAC address learning spans both the datapath and control plane. The Forwarding Datapath Unit (FDU) has to check the source MAC address in every received packet against the forwarding table. If the source MAC address does not exist in this table, the source MAC address must be sent to the control plane for learning purposes. This will ultimately result in adding the MAC address to the forwarding table in the FDU. Examples of current implementations for missed source MAC address include sending a copy of the packet to the control plane and sending just the source MAC address to control plane.